High-purity quartz (HPQ) is a crucial material in various high-tech industries, including electronics, optics, and photovoltaics. The purification of quartz is essential to achieving the necessary purity levels required for these applications. Quartz purification methods are broadly classified into physical and chemical techniques. Each method targets specific impurities and plays a vital role in the refinement process.
Physical Purification Methods
Physical purification techniques primarily remove mineral impurities present in quartz as monomers. The most commonly employed physical methods include:
Scrubbing: This mechanical process helps to remove surface impurities and thin film contaminants.
Gravity Separation: Utilizes the density differences between quartz and impurities to achieve separation.
Magnetic Separation: Effectively eliminates iron-containing impurities using magnetic fields.
Color Separation: Distinguishes quartz particles based on their optical properties to remove visually distinct impurities.
Flotation: Employs chemical reagents to selectively separate quartz from unwanted minerals.
These methods effectively reduce impurity levels, but they are insufficient for removing gas-liquid inclusions and homogeneous impurities within the quartz crystal lattice.
Challenges in Achieving Ultra-High Purity
Even after rigorous physical purification, certain impurity elements, such as alkali metals (K and Na), remain embedded within the crystal lattice. These impurities negatively impact the performance of quartz glass by lowering its service temperature, reducing mechanical strength, and accelerating devitrification at high temperatures. Physical methods alone cannot eliminate these deep-seated impurities, necessitating advanced chemical purification techniques.
Chemical Purification Techniques
Chemical purification processes play a crucial role in the deep refinement of quartz. The primary methods include:
Acid (Alkali, Salt) Treatment: Removes surface and embedded impurities through dissolution in strong acid or alkali solutions.
Heat Treatment: Utilizes high temperatures to volatilize impurities, including isomorphic substitutions within the quartz lattice.
While chemical purification methods are more complex and expensive than physical techniques, they are indispensable for producing high-purity quartz. Among these, chlorination roasting has emerged as the most effective technique for removing interstitial atomic impurities and alkali metals.
Chlorination Roasting: The Key to High-Purity Quartz
Chlorination roasting is a widely adopted method for purifying quartz by converting impurity ions into volatile chlorides that can be efficiently removed. The process involves heating quartz sand in the presence of a chlorinating agent under controlled atmospheric and temperature conditions.
Commonly used chlorinating agents include:
Chlorine (Clfl)
Hydrogen chloride (HCl)
Ammonium chloride (NH₄Cl)
Sodium chloride (NaCl)
Potassium chloride (KCl)
Calcium chloride (CaCl₂)
The chlorination roasting process can be categorized into different temperature-based methods:
High-temperature roasting (Chlorination volatilization method): Operates at extreme temperatures to directly volatilize impurities.
Medium-temperature roasting (Chlorination roasting-leaching method): Involves moderate heating followed by impurity dissolution through leaching.
Chlorination-isolation: Focuses on selective removal of specific elements.
Traditional Chlorination Roasting Technology
In conventional chlorination roasting purification, quartz sand is typically heated to 1000-1200°C in a Cl₂ or HCl gas atmosphere. At these elevated temperatures, trace metal elements such as Al³+, Fe³+, and Ti⁴+, which are bound within the quartz lattice, diffuse towards the surface. These elements then react with Cl- ions to form volatile chlorides, which are swiftly eliminated, significantly enhancing the purity of quartz.
Conclusion
The purification of high-purity quartz is a multi-stage process that involves a combination of physical and chemical methods. While physical purification effectively removes external mineral impurities, chemical purification, particularly chlorination roasting, is indispensable for achieving ultra-high purity levels. Continued research and technological advancements in quartz purification are critical for meeting the ever-growing demands of high-tech industries.
